The rollout of universal credit to families with children should be paused, the Children’s Commissioner for England has told MPs.
Anne Longfield told the Education Select Committee there was evidence families with children were being hit hard by the welfare changes.
The impact of universal credit had not been tested on families with children, said Ms Longfield.
The government says the change will make it easier to claim benefits.
The rollout of the new benefit across the UK accelerated last month – with about 50 job centres now being added each month.
It merges six benefits for working-age people into one new payment.
What is Universal Credit?
Universal Credit: Helpline delays
The system, with a built-in six-week wait, has been beset by controversy.
The benefit is paid in arrears, which means everyone has to wait at least four weeks for their money.
The rest of the wait is because of the way the scheme is administered.
So far, about a quarter of all claimants have had to wait more than six weeks to receive their first payments.
“I am worried about the rollout of welfare reforms,” Ms Longfield said in her evidence to the committee.
“I am aware that, actually, families with children are being very hard hit, families with more than two children very hard hit, and, actually, lone parents.
“So, I do think we’ve got a set of families whose lives are quite precarious.
“Often they are the ones in work, and I am not sure that everyone has recognised that the new poor and the new insecure are those in insecure work.”
She said the fact that the new system had not been tested on families with children meant “we are moving into rollout not knowing what that means”.
“So, one of the things that I will be asking the chancellor to do is to pause universal credit rollout for families with children until we better understand what that means,” Ms Longfield said.
She said the change was coming on top of “welfare failures” and families were now having to resort to using food banks.
“I am well aware of the impact of the rules around two children and also the benefits freeze,” she said.
“There is cumulative impact and I think at this stage, with such a vulnerable group of children and families, it’s the right thing to pause universal credit.”
The Department for Work and Pensions says universal credit will boost employment by about 250,000 once it is fully rolled out.
A spokeswoman said: “Under universal credit, people are moving into work faster and staying in work longer than the old system.
“The number of children growing up in homes where no-one works has fallen by half a million since 2010.
“Under UC, parents get tailored support to find work that fits with their caring responsibilities and, once in work, have 85% of their childcare costs refunded.”